Regardless if he is using or not it sounds as though he is not being respectful of your marriage. If you were uncomfortable with him going on a trip with his friends regardless of who they are then that should of been disgust. In a healthy marriage you must talk and come to conclusions that sit well with both parties.
IMO if the buddies he was going on the trip with were known users he wouldn't of put himself in that position to begin with. I believe his sponsor would of talked to him about whether or not that was the best situation for him to be in esp after only 100 days clean.
Does your gut say he's using again? Does his actions match his words?
Addicts are great at passing drug tests. I would keep your eyes open! Normally when it quakes like a duck its a duck!
What are you doing for you? You can't control him . . . you can only control how you react to him!